It is well-known that in order to conform to legal standards related to the allowed exhaust gas emissions of an internal combustion engine a high efficiency of exhaust gas purification methods is required. One of these methods is a preferably precise adjustment of the exhaust gas composition such that a catalytic converter disposed in the exhaust gas system preferably may operate effectively. In order to achieve a high conversion efficiency in today's three way catalytic converters these are charged with exhaust gas which alternatingly has a slight fuel surplus (rich) or a slight oxygen surplus (lean). According to prior art this so called lambda modulation is controlled by the measurement signal of a lambda sensor installed upstream of the catalytic converter. For the purpose of monitoring, installed downstream of the catalytic converter frequently is a second lambda sensor, whose measurement signal provides information about the achieved efficiency of the controlled exhaust gas system and for example allows for a closed loop system. It may be assumed that this downstream monitoring lambda sensor is aging less intense or fast due to the position more distant from the engine, and overall and as seen across the product life supplies a considerably more precise measurement signal due to the exhaust gas composition already reacted downstream of the catalytic converter. Therefore, the rearmost lambda sensor is used for correcting the forward lambda control and/or for adapting signal deviations of the upstream lambda sensor.
Today's lambda sensors are based on the operating principle that ceramics become conductive for oxygen at high temperatures. Therefore, the known lambda sensors for example have a ceramic body on which electrodes for determining a voltage or pumping electricity are applied, as well as a heating element which heats the ceramic body to temperatures in the range of 600-800° C. However, if fluid water arrives at the hot ceramic body at these temperatures then there exists the risk of damaging the element as a result of the thermal stress arising thereby. For this reason, according to prior art it is typically awaited with the heating of the lambda sensors until assuredly no fluid water caused by condensation or stratification can be present anymore at the installation position of the lambda sensor. Applicable calculation functions typically are located in an engine control unit. The problem arising therefrom is that the lambda sensors can only be heated some time after an engine start and until then the engine only can be operated in an uncontrolled manner which results in a degradation of the exhaust gas emission. This is particularly critical for the rearmost lambda sensor because the more distant the installation position is from the engine the longer it takes until the required temperature is reached at which no fluid water is present anymore (so called dew point stop). It would therefore be desirable to be able, already at an early point in time during the cold operating phase of an internal combustion engine before achieving the dew point stop of the exhaust gas equipment, to provide to the exhaust gas control an evaluable signal of the lambda sensor.
DE 10 2006 011 722 B3 discloses a method for correcting the output signal of a broad band lambda sensor of an internal combustion engine. Within the scope of this method the influence of humidity on the lambda value determined by the broad band lambda sensor is identified and computationally eliminated by means of a compensation model. For this purpose a measured humidity is introduced in the calibration of the broad band lambda sensor during an overrun fuel cut-off of the internal combustion engine.
DE 10 2005 059 794 B3: After switching form a presetting of a rich fuel/air mixture ratio in a combustion chamber of a respective cylinder of an internal combustion engine to a presetting of a lean fuel/air mixture ratio it is detected for a thereupon arising plateau phase of a measurement signal of an exhaust gas sensor disposed in an catalytic exhaust gas converter and this time period is determined as to be the emplacement time period. After switching form a presetting of a lean fuel/air mixture ratio in the combustion chamber of the respective cylinder to a presetting of a rich fuel/air mixture ratio a thereupon arising plateau phase of the measurement signal is detected and the time period of the plateau phase is determined as the release time period. Depending on the accumulation time period and the depletion time period an assignment rule for assigning the measurement signal to a detected fuel/air mixture ratio is adapted. In order to calibrate the exhaust gas sensor the assignment rule is adapted depending on a plateau value of the measurement signal during the plateau phase.
The following patent documents related to the technological background of the present invention are known: DE 10 2006 011 722 B3, DE 103 60 775 A1, DE 198 61 198 B4, DE 43 04 966 A1, DE 199 37 016 A1, DE 10 2004 006 875 A1, DE 103 39 062 A1, DE 199 26 139 A1 and DE 10 2005 038 492 A1.
Known from DE 199 34 319 A1 is a gas measurement sensor which has a protective pipe for protecting the ceramic sensor element. A further inner pipe comprising openings for entrance and exit of the measurement gas and the exhaust gas, respectively, is meant to protect the ceramic sensor element against a direct contact with water.
According to DE 10 2004 020 139 A1 a lambda sensor for an internal combustion engine for measuring the fuel/air mixture ratio in the exhaust gas flow of the internal combustion engine comprising an oxygen sensor element is proposed in which the portion of the oxygen sensor element extending into the exhaust gas flow is encompassed by a protective element for collecting condensation water. The lambda sensor constructed such may be put into operation already before or instantaneously after the start of the internal combustion engine since the risk of cold condensation water impacting the hot oxygen sensor element and the damage of the lambda sensor associated therewith shall be eliminated.
Known from DE 10 2004 035 230 A1 is a method for operating a gas measurement sensor by means of which operating states of the internal combustion engine are determined. Upon existence of an operating state in which a low temperature is to be expected in the exhaust gas line, as for example at a cold-start, the sensor is adjusted to a low temperature or is turned off completely in order to counteract the risk of a thermal shock due to the reaction to water. The sensor therefore does not have an adjustment ability at the start of the internal combustion engine.
According to DE 10 2004 054 014 A1 a ceramic component, in particular a sensor element for a gas sensor, for determining a physical characteristic of a measurement gas, in particular the temperature or the concentration of a component of the gas in the exhaust gas of internal combustion engines is specified which has a, in particular laminated, ceramic body. For a significant improvement of the thermal shock behavior of the ceramic body, i.e. for obtaining a significantly lowered sensitivity with respect to the occurrence of strongly localized temperature gradients, which initiate crack formation in the ceramic body, at least the surface areas of the ceramic body which are exposed to large temperature gradients are coated by a protective coating which has at least two ceramic layers which form an intermediate boundary layer comprising a low fracture energy.
DE 10 2006 012 476 A1 discloses a method for operating a sensors, in particular a sensor comprised of a ceramic material, wherein the sensor is heated up to a shock resistance temperature which is greater than a specified operating temperature of the sensor. After also the vicinity of the sensor has been heated by the shock resistance temperature for some time the normal operating temperature is adjusted. It is further proposed to at first regulate a temperature lower than the normal operating temperature.
DE 10 2004 031 083 B3 discloses a method for heating lambda sensors in an exhaust gas system arranged downstream of the internal combustion engine of a vehicle comprising at least one catalytic converter equipment in the exhaust gas line of the exhaust gas system as well as comprising a sensor disposed upstream of and downstream of the catalytic converter, respectively, wherein in order to avoid a water ingestion risk for the sensors the heating of the sensors to their operating temperature is started at a heating time at which a predefined condensation formation temperature critical for the condensation formation in the region of the exhaust gas line is exceeded. At a cold-start of the internal combustion engine, starting at a predefined heating time, out of the two sensors at first only the downstream sensor is heated to a predefined sensor temperature. The sensor heated to this temperature, in the further course of the cold-start phase, for a time period until a condensation formation temperature critical to the condensation formation in the upstream region of the exhaust gas line is exceeded is operated by a control device as a control sensor by means of which the control of the lambda value is carried out to reach a predefined lambda value. Upon overstepping the critical condensation formation temperature in the pre-catalytic converter region of the exhaust gas line the upstream sensor is heated up to a predefined sensor temperature. The method disclosed necessarily uses one lambda sensor upstream of the catalytic converter and one lambda sensor downstream of the catalytic converter. This limits the use of the method to exhaust gas systems comprising two lambda sensors, whereby increased cost and an additional technical sensitivity have to be accepted.